The present invention relates to the continuous processing of scrap tires, plastic and rubber wastes, and/or other polymeric materials into valuable chemicals and/or fuels.
In the world today, waste polymeric materials, especially spent and discarded vehicle tires litter the landscape of the developed countries, causing serious environmental and economic problems. In the USA alone, over 3 billion scrap tires have accumulated in the open air, with at least one location containing 30 million tires (“Markets for Scrap Tires” issued by the US Environmental Protection Agency, Office of Solid Waste, 1991). The landfills containing scrap tires are unsightly and expose ground and atmosphere to dangerous pollution. There are also several additional aspects, which raise a general public awareness associated with the dumps containing scrap tires. These include:                (i) Fire Hazard        
Discarded tire dumps are prone to fires and once such a dump bursts into flames it will continue burning for months, creating acrid smoke and leaving behind a hazardous oily residue.                (ii) Expensive Disposing of Waste Tires in Landfills        
The tipping fees for disposal of tires have trended upwards with continuous increases. For example, in the Los Angeles area, the tipping fees in 2007 have already been set to be about 10% higher compared to those in 2006 and make up $66.9 per ton.                (iii) Source of Mosquito Breeding        
Because of the bulky and impermeable structure of tires piled in dumps, they may hold water for a long period providing exceptionally inviting breeding sites for mosquito larvae, which when developed into mosquitoes sustain the spread of diseases.                (iv) Deficiency of Land and Tightening of Environmental Regulations        
For the purposes of tire disposal, it continually becomes more difficult to expand old landfills or find new sites for them.
In order to solve the problem concerning the waste tire accumulation, scrap tires should be recycled or reused. One of the recycling options is the chemical decomposition of tires in the course of which valuable products can be manufactured.
From a chemical point of view, a tire is a polymeric composite reinforced by both polymeric fibers and a steel cord. It is noteworthy that the requirements on the tire composition vary from producer to producer and depend upon the type of vehicles or industrial and other machines for which tires are destined. An example of the most common compounding ingredients is given in Table 1, below.
TABLE 1Example of tire composition*Compound basicWeightcompositionMain compounding ingredientspercentRubber49.8%Natural rubber27.7%Synthetic rubber22.1%FillerCarbon black26.1%SilicaReinforcing cordTextile (rayon, polyamides4.1%(e.g. nylon), polyester, aramidfibers etc.)Steel cord and bare wire14.1%Compounding ingredients5.9%Vulcanizing agentSulfur, organic compoundsVulcanizingThiazole-type acceleratorsacceleratorPromotorZinc oxide, stearic acidAntioxidantAmines, phenolic compounds,waxesSoftenerPetroleum-type oil, aromatic oil*The source: Ullmann's Encyclopedia of Industrial Chemistry, 5th electronic edition
It is necessary to point out that the tire material of today's tires comprises nearly all types of polymeric compounds that can also be found in other plastic wastes. This can be seen from Table 1. From this point of view, the present invention discloses the processing methods suitable for any plastic and rubber wastes as well as other polymeric materials.
It is understood that as used herein, the terms “tires or scrap/spent/worn/discarded/used/old/waste tires,” and the like are used interchangeably and refer to the utilization/recycling/decomposition/processing/treatment of both scrap tires and other polymeric/plastic/rubber wastes.
It should be noted that the decomposition of scrap tires in the continuous operation mode is much more complicated than the continuous processing of other polymeric wastes because of a significant content of carbon (and steel) that cannot be completely converted into gaseous or liquid products and, therefore, should be permanently removed out of a decomposition reactor.
The decomposition processes of scrap tires mentioned in the available patent and scientific literature can be ranged into the following classes: pyrolysis, gasification and dissolution. It is necessary to point out that the reaction mechanism in all these processes has similar features so that the conventional classification is to some extent relative.
Conventionally, pyrolysis implies the process in which organic substances are reduced or cracked by subjecting a material to heat in the absence of oxygen. The pyrolytic reactions are endothermic, i.e. they demand a delivery of heat to a reactor. In general, the pyrolytic process can reduce scrap tires into three product streams: a liquid, a gas and carbon char.
U.S. Pat. No. 5,821,396 dated Oct. 13, 1998, of Bouzlane, entitled “Batch process for recycling hydrocarbon containing used materials”, describes a process wherein pyrolysis of used tires is carried out in a batch rotating reactor under atmosphere of an inert gas in the range of 435-500° C. However, this method has distinct drawbacks related to the complex equipment and reactor design and to a batch operation mode necessitating the stages of the periodic heating and cooling of the reactor. The system also has to be vacuumed and then pressurized with an inert gas after every charging. It can be expected that batch operating demands cumbersome equipment not well suited for industrial needs.
Gasification processes comprise very complex reactions occurring during partial combustion (or oxidation) of scrap tires and simultaneous cracking of organic molecules. Gasification is characterized by considerably higher temperature than in pyrolytic processes and does not demand a heat supply. As a result, the products of gasification processes are gaseous and preferably used for heat generation.
The patent family of German Patent No. DE 10051246 issued on May 23, 2001, Russian Patent RU No. 2186295 issued on Oct. 27, 1999 and U.S. Pat. No. 6,321,666 dated Nov. 27, 2001, entitled “Cycle Installation for Utilization of Waste Containing Polymer Compounds”, inventor Vladimir Tigonen, describes the typical gasification process carried out at high temperatures up to 1400° C. The technical object of that prior development is heat generated by the combustion of the tires. The gasification assembly requires a very large tank operated in a batch mode.
In general, gasification processes need a rigid connection to heat generation facilities installed on the same site. Otherwise, a gasification plant should be accompanied by a liquefaction station for gas storage and its transportation.
Dissolution of scrap tires is similar to the pyrolysis process. The scrap tire decomposition is carried out in a liquid medium in which shredded tires are submerged. In order to prevent evaporation of this liquid medium, the process runs under high pressure.
The patent family of International Patent No. WO2004094562 issued on Jan. 10, 2004, European Patent Application No. EP 1632546 and Russian Patent No RU 2220986 dated Jun. 10, 2004, entitled “Method of Processing Rubber-Containing Wastes”, inventor Kiril Z. Bochaver, describes the method for destruction of rubber waste. The process is carried out at temperatures of 280-435° C. and pressures of 29-60 bar in an organic solvent that can be extracted from the liquid product and returned into the process.
The liquid, gas and solid compounds produced according to Bochaver have almost identical chemical properties and weight content as during the pyrolysis realized under the same temperatures.
Unfortunately, the time of the reaction is not specified in Bochaver's description. From practice it is known that the residence time is about 1 hour for the batch performance. Despite the fact that Bochaver refers to the reactor working in a continuous operation mode, this part of Bochaver's description has not been verified from a practical standpoint. All technical illustrations in this patent are presented by experiments brought about in a batch operation mode.
The commercial development of this prior art can encounter many technical difficulties especially related to the continuous process in the reactor under high pressure, such as: (a) the feed of the bulky shredded tires, (b) the removal of a solid product (carbon char) from the reactor, and (c) the withdrawal of the residual liquid remaining in this porous solid. Even if these problems were to be solved, the design of an industrial unit would be very complex and the reactor itself would be cumbersome. On the whole, solving all these problems will result in high capital and operation costs.
In applicants' view, there have been no realistic technical solutions appropriate for wide industrial applications. Despite the fact that compounds produced from the scrap tires are identical to those processed from crude oil and can be used for both the recycling of valuable chemicals and energy generation, the above-mentioned prior processes cannot thus far compete with petroleum refinery operation at today's prices. Applicants do not know of any example of successfully operated plants on an industrial scale.
It is believed the difficulties of an industrial operation proceed from the complicated and cumbersome reactor design, deposition of carbon as a by-product on the internal surfaces of the reactor, which tremendously worsens heat transfer, the necessity of complex feeding, discharging and transporting systems, and in the case of scrap tires, high costs of the further carbon recovery. All these factors result in high investment and operating costs.